1) Field of the Invention
This invention relates to an improvement in a spin dryer to remove water droplets stuck on the surface of semiconductor wafers by centrifugal force and to dry the same and, in particular, to a filter-box positioned on such a spin dryer.
2) Description of the prior art
This kind spin dryer comprises of a casing and a rotor rotated in the casing. Semiconductor materials to be dried, usually called wafers, are housed in a carrier, the carrier being in turn housed in a cradle to be set in a predetermined position on the rotor. There is provided a lid having a suction port formed therein, with a filter or a filter-box disposed above the rotor. The casing has an exhaust port at a suitable position in its peripheral wall. Rotation of the rotor generates a negative air pressure in the central zone of the rotor, which draws air through the filter into the rotor, so the air is filtered. The introduced air flows through the rotor to the peripheral wall of the casing and flows out through the exhaust port. Such an air stream dries the wafers set on the rotor.
In this process, the wafers have a static electric charge which has undesirable influence on the semiconductor wafers so that the wafers are apt to attract dust or foreign particles.
In the commonly accepted technique for removing such static electricity, for example, an arrangement disclosed in Japanese U. M. Laid-open gazette No. 62433/1987 is used. This known arrangement shown in the above described gazette is shown in FIG. 8. There are some corona electrodes (c) located at suitable positions on the periphery of the suction port lid above the rotor. The electrodes serve to ionize the air introduced through the filter (d) of the suction port in order to remove static electricity on the wafers. However, according to this arrangement, air passing through the central area of the suction port and through the areas between the electrodes will not be ionized, and thus the amount of ionized air is not enough.
Also, another arrangement in the prior art such as is shown in FIG. 9 was known, in which a filter-box (e) is located above the rotor. This known filter-box has an annular filter (f) and ionizers (g) which consist of corona electrodes that are equally spaced on the inner periphery of the filter. Although this type of filter-box has merit in that it can accommodate a high quality filter such as the so-called Hepafilter or Ulpa-filter, it is accompanied with some problems which are described below.
Usually, in the process of spin drying, static electricity of 30,000-50,000 Volts is generated on the semiconductor wafers. As shown in FIG. 10, there are two streams of introduced air, one of which passes in touch with or near to an ionizer (g) so thereby is ionized, but the of which does not pass near to an ionizer so that it is not ionized. Accordingly, the amount of ionized air is not enough, which results in the incomplete removal of static electricity from the wafers. This leads to a reduction of the above voltage value of static electricity to 5,000-10,000 Volts. In addition thereto, since the lower surface of the ceiling plate of the prior art filter-box is a flat plane there is a stagnant space just under the central area of the lower surface ceiling, Thus, this stagnation of air in the central area accumulates fine dust or particles that pass through the filter, on the lower surface of the ceiling plate. Such accumulated dust will eventually be separated from the lower surface and be attached to the wafers, set in the rotor and thereby stain the same.